1. Field of Invention
This invention relates to a network of data stations and more particularly to such a network wherein control of the transmission between stations is shared by the respective stations.
2. Description of the Prior Art
Prior art terminal networks usually have been controlled by a master computer which receives transmission requests from the respective terminals and grants access to a transmission channel by the individual terminals when the channel is available and according to some priority arrangement. Such master computers add to the cost of the terminal network and are not required for some networks where the terminals need communicate only between themselves or with a common storage file. Thus, it is desirable to have a terminal network where the transmission control is embedded in or shared by the stations making up that network.
Prior art networks not having a master controller, have employed "contention" schemes whereby each node of the network contends for the transmission medium whenever it is ready to transmit. One of the earliest contention networks was the "Aloha" system built by the University of Hawaii. In this system, each node transmitted whenever it had a packet ready for transmission. Whenever a portion of one node's transmission overlapped with another node's transmission, that overlap destroyed both packets. If the sending node did not receive an acknowledgment within another packet from the destination node after an arbitrary time period, it would assume that a collision had occurred and retransmit. In order to avoid continuously repeated collisions, some method of introducing a random retransmission delay had to be introduced.
An attempt at reducing the effects of collision in contention-type networks is disclosed in the Metcalfe et al. U.S. Pat. No. 4,063,220 which is directed toward a communication network in which each node is capable of detecting when collision occurs on the channel during that node's transmission, and interrupts its transmission when such a collision occurs. A random number generator is employed to select an interval of time to delay before the next attempt at transmission. However, the collision detection mechanism adds to the complexity of the respective nodes with an increase in the cost of the system. For certain types of transmission media, collision detection is very difficult, if not impossible to implement.
It is, then, an object of the present invention to provide an improved data communication network which does not require a master controller to synchronize data transmissions on a common bus or channel.
It is another object of the present invention to provide an asynchronous data communication network wherein each node contends for access to a channel medium in a manner such as to minimize conflicts between the respective nodes without requiring transmitting stations to be capable of detecting collisions.
It is still another object of the present invention to provide a contention network data communication system wherein the respective nodes contend for access to the channel in a cyclic fashion with synchronization between the states of each node.
It is still a further object of the present invention to provide a contention network data communication system wherein an acknowledgment signal is received (or not received) by the transmitting station, within a predetermined period of time after transmission.